1. Field of the Invention
This invention relates to improvements in circuitry for sensing and amplifying electronic signals, and more particularly to a circuit that uses a sensefet to sense the current flowing through the stator coil of a polyphase motor and a fast amplifier for amplifying the sensefet output voltage, and still more particularly to improvements in motor driver circuits for polyphase dc motors.
2. Description of the Prior Art
Although the present invention is potentially useful for a variety of current-sensing applications, it finds particular application in the operation of three-phase dc motors of the type typically used in computer-related applications. These computer applications include hard disk drives, CD ROM drives, floppy disks, and the like, in which three phase brushless, sensorless dc motors are becoming more popular, principally due to their reliability, low weight, and accuracy.
To control the motor speed, a transconductance loop is usually used in the driver stage to obtain good regulation. Therefore, a need exists to measure accurately the current through each coil of a polyphase motor.
One device that has been recently advanced to perform a measuring function in other applications is the sensefet, a field-effect transistor designed for current sensing, particularly with high power loads. A sensefet typically presents an output, called "sense," that provides a current proportional to the current in the main device. When a sensefet approach is used to monitor current on a power conductor, it may be important for the particular application to have a very fast amplifier to convert the "sense" current into a voltage signal for further processing. For example, amplifier speed is important when the amplified voltage signal is used as feedback for a motor control loop. In motor control loops, an appreciable delay in the signal may cause a significant error in the output of the loop.
What is needed is a circuit that will quickly respond to the input voltage presented on the output of an N-channel D-MOS FET or similar device when that device is used as a "high-side" driver (i.e., one that sources current to the load). The output voltage is generally available on a resistor connected to a reference potential or to the lower supply rail.
Until now amplifiers built in a standard power integrated circuit technology have been too slow to be useful for feedback loops, and have had bandwidths that are too small. Fast single-gain-stage amplifiers have had gains that are too low to yield acceptable operating precision, and either would not operate or have not had a fast recovery when overload voltages have occurred. (An overload voltage occurs when voltage to be sensed is greater than the amplifier supply voltage V.sub.CC.)
Accordingly, the amplifier should have an input common mode voltage range compatible with (i.e., greater than) the supply voltage, to prevent saturation of the input stage. The amplifier should be a single-gain-stage amplifier. Therefore, no compensation capacitors are required and the device can be easily and inexpensively integrated onto an integrated circuit device. Also, it should be easy to connect the amplifier in parallel with other amplifiers of the same type. In this way, the current can be sensed on multiple stages and summed, giving a measure of the total current drawn by the stator coils at any particular time.